The present invention relates to an optical glass having optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (xcexdd) having a range of 21 to 28, wherein a light transmittance is excellent to a short wavelength band. In particular, the present invention relates to an optical glass having an excellent light transmittance to a short wavelength band, and optical constants which are a refractive index (nd) having a range of 1.80 to 1.87 and an Abbe number (xcexdd) having a range of 21 to 26, and having a photoelastic constant (xcex2) having an absolute value of not more than 0.2xc3x9710xe2x88x925 nm/cm/Pa in case of e-line (wavelength 546.07 nm), the optical glass being especially suitable for being used for a spatial light modulation element for carrying out a polarization modulation or for a polarization optical system, such as a beam splitter or the like.
In recent years, an optical system utilizing a polarization, that is, a polarization optical system is used in various fields, such as a liquid crystal projector or the like. For example, a spatial light modulation element for spatially modulating a polarization, a polarizing beam splitter for separating a light into S-polarization and P-polarization and the like are used for a liquid crystal projector or the like. In these polarization optical systems, it is desired to control the properties of polarization at a high accuracy.
In optical parts, such as a substrate, a prism for a polarization optical system, or the like, when a material having an optical anisotropy is used in a part which is required to keep the properties of polarization, a phase difference (optical path difference) between a transmitted principal ray and an extraordinary ray perpendicular to the principal ray is changed as compared with the difference before a principal ray is transmitted through the material. Because the properties of polarization cannot be kept, it is necessary that a material having an optical isotropy should be used for these parts.
A glass in which strains are sufficiently removed, in particular, an optical glass has an optical isotropy. Further, the durability, the strength and the light transmittance of the optical glass are more excellent than those of the other materials. There are various types of optical glasses having various optical constants. The optical glasses are more selective in an optical design. Therefore, the optical glasses have been used for a polarization optical system before. In particular, S-BSL7 (which is a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara) is inexpensive and has a good durability. Further, the dispersion thereof is small. Therefore, the S-BSL7 is often used in a polarization optical system.
However, even though the above-mentioned optical glass having an optical isotropy according to an earlier development was used in an optical part of a polarization optical system, the optical glass has an optical anisotropy caused by the photoelastic effect when mechanical stress and thermal stress are applied to these parts, that is, has a double refraction property. As a result, there was a problem that it is difficult to obtain desired properties of polarization. The above mechanical stress is caused, for example, by joining a material having a coefficient of thermal expansion which is different from that of the glass, to the glass. The above thermal stress is caused, for example, by generating heat from peripheral devices, or by generating heat from the glass itself because of absorption of energy of a transmitted light. An amount of the double refraction which the glass induces by applying these stresses to the glass, can be represented by using the optical path difference. When xcex4 (nm) is the optical path difference, d (cm) is the thickness of the glass and F (Pa) is the stress, the following equation (1) holds. The equation (1) means that the more the optical path difference increases, the more the double refraction increases.
xcex4=xcex2.d.Fxe2x80x83xe2x80x83(1)
In the equation (1), the proportional constant (xcex2) is called photoelastic constant. The values thereof vary in type of glass. As shown in the equation (1), when the stress (F) applied to the glass and the thickness (d) of the glass are constant, the smaller the absolute value of the photoelastic constant (xcex2) of the glass is, the shorter the optical path difference (xcex4) is, that is, the smaller the double refraction is. In the above S-BSL7 (which is a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara), the value of the xcex2 is 2.79xc3x9710xe2x88x925 nm/cm/Pa in case of e-line (wavelength 546.07 nm), and is large. In order to control the properties of polarization at a high accuracy in the polarization optical system as described above, the glass is required to have a small absolute value of the photoelastic constant (xcex2).
In an earlier development, as a glass having a small photoelastic constant xcex2, PBH53 (which is a trade name of the optical glass manufactured by Kabushiki Kaisha Ohara) having a composition of SiO2xe2x80x94PbO system wherein the refractive index (nd) is about 1.847 and the Abbe number (xcexdd) is 23.9, and equivalent glasses manufactured by other companies, have been known. These glasses have sufficiently small photoelastic constants (xcex2) of less than 0.1xc3x9710xe2x88x925 nm/cm/Pa in case of e-line (wavelength 546.07 nm). However, in a short wavelength band of not more than 450 nm, the light transmittance cannot be sufficiently obtained. Because a difference between the intensities of three separated lights, blue (B), green (G) and red (R) which are used in a liquid crystal projector or the like, is caused, it is necessary that the intensities of the lights are adjusted in accordance with the blue light (B) having a low intensity. As a result, there is a problem that an amount of light to be projected from a liquid crystal projector or the like, is not sufficient.
An object of the present invention is to provide an optical glass having optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (xcexdd) having a range of 21 to 28, wherein a light transmittance is excellent to a short wavelength band, in consideration of the above circumstances of the earlier development. In particular, an object of the invention is to provide an optical glass having optical constants which are a refractive index (nd) having a range of 1.80 to 1.87 and an Abbe number (xcexdd) having a range of 21 to 26, having an excellent light transmittance to a short wavelength band, and having a photoelastic constant (xcex2) having a small absolute value.
In order to accomplish the object, the inventors have examined and researched an optical glass. As a result, the inventors have found that an optical glass having optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (xcexdd) having a range of 21 to 28, wherein a light transmittance is excellent to a short wavelength band as compared with that of an optical glass according to an earlier development, can be obtained by adding the component TeO2 to a glass having a composition system including SiO2 and PbO. Further, the inventors have found that an optical glass having optical constants which are a refractive index (nd) having a range of 1.80 to 1.87 and an Abbe number (xcexdd) having a range of 21 to 26, wherein a light transmittance is excellent to a short wavelength band, and an absolute value of a photoelastic constant (xcex2) is small, can be obtained. Then, the present invention has been accomplished.
That is, in order to accomplish an object of providing an optical glass having optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (xcexdd) having a range of 21 to 28, wherein a light transmittance is excellent to a short wavelength band, in accordance with the first aspect of the present invention, an optical glass comprises the following composition in mass percent of: SiO2 18-29%, PbO 68-78%, and TeO2 0.1-3.5%; wherein the glass has optical constants which are a refractive index (nd) having a range of 1.75 to 1.87 and an Abbe number (xcexdb) having a range of 21 to 28; and a wavelength of a light transmitted through the glass having a thickness of 10xc2x10.1 mm at a transmittance of 80% by including a reflection loss, is not more than 420 nm.
Further, the optical glass according to the first aspect of the present invention, may comprise the following composition in mass percent of: SiO2 18-29%; PbO 68-78%; TeO2 0.1-3.5%; B2O3 0-6%; R2O 0-5%, the R2O being one or more selected from the group consisting of Li2O, Na2O and K2O; Rxe2x80x2O 0-5%, the Rxe2x80x2O being one or more selected from the group consisting of SrO, BaO and ZnO; GeO2 0-5%; Al2O3 0-3%; Nb2O5 0-3%; In2O3 0-3.5%; Ga2O3 0-3.5%, a total amount of In2O3+Ga2O3 being 0-3.5%; As2O3 0-1%; Sb2O3 0-1%; and a total amount of F elements of one or more fluorides which are partially or wholly substituted for one or more oxides of each metal element described above 0-2%; wherein the optical glass comprises the GeO2, the Al2O3, the Nb2O5 and the F elements in a total amount of 0-5%.
According to the first aspect of the present invention, in the optical glass, an amount of the R2O may be in a range of 0 mass % to less than 0.3 mass %, the R2O being one or more selected from the group consisting of Li2O, Na2O and K2O.
According to the first aspect of the present invention, in the optical glass, an amount of the R2O may be in a range of 0 mass % to less than 0.1 mass %, the R2O being one or more selected from the group consisting of Li2O, Na2O and K2O.
In order to accomplish an object of providing an optical glass having optical constants which are a refractive index (nd) having a range of 1.80 to 1.87 and an Abbe number (xcexdd) having a range of 21 to 26, wherein a light transmittance is excellent to a short wavelength band and the optical glass has a photoelastic constant (xcex2) having a small absolute value, in accordance with the second aspect of the present invention, an optical glass comprises the following composition in mass percent of: SiO2 18-27%; PbO 71-78%; and TeO2 0.2-3.5%; wherein the glass has optical constants which are a refractive index (nd) having a range of 1.80 to 1.87 and an Abbe number (xcexdb) having a range of 21 to 26; the glass has a photoelastic constant (xcex2) having an absolute value of not more than 0.2xc3x9710xe2x88x925 nm/cm/Pa in case of e-line (wavelength 546.07 nm); and a wavelength of a light transmitted through the glass having a thickness of 10xc2x10.1 mm at a transmittance of 80% by including a reflection loss, is not more than 420 nm.
Further, the optical glass according to the second aspect of the present invention, may comprise the following composition in mass percent of: SiO2 18-27%; PbO 71-78%; TeO2 0.2-3.5%; B2O3 0-6%; R2O 0-5%, the R2O being one or more selected from the group consisting of Li2O, Na2O and K2O; Rxe2x80x2O 0-5%, the Rxe2x80x2O being one or more selected from the group consisting of SrO, BaO and ZnO; GeO2 0-5%; Al2O3 0-3%; Nb2O5 0-3%; In2O3 0-3.5%; Ga2O3 0-3.5%, a total amount of In2O3+Ga2O3 being 0-3.5%; As2O3 0-1%; Sb2O3 0-1%; and a total amount of F elements of one or more fluorides which are partially or wholly substituted for one or more oxides of each metal element described above 0-2%; wherein the optical glass comprises the GeO2, the Al2O3, the Nb2O5 and the F elements in a total amount of 0-5%.
According to the second aspect of the present invention, in the optical glass, an amount of the R2O may be in a range of 0 mass % to less than 0.3 mass %, the R2O being one or more selected from the group consisting of Li2O, Na2O and K2O.
According to the second aspect of the present invention, in the optical glass, an amount of the R2O is in a range of 0 mass % to less than 0.1 mass %, the R2O being one or more selected from the group consisting of Li2O, Na2O and K2O.
According to the second aspect of the present invention, the optical glass may have the photoelastic constant (xcex2) having the absolute value of not more than 0.1xc3x9710xe2x88x925 nm/cm/Pa in case of the e-line (wavelength 546.07 nm).
Next, in an optical glass according to the first aspect of the present invention, the reason why a composition range of each component is limited as described above, will be explained.
The SiO2 component is an essential component for forming a glass. However, when the SiO2 component is present in an amount of less than 18%, the refractive index of the optical glass is too high and the light transmittance is not sufficient. When the SiO2 component is present in an amount of over 29%, the refractive index becomes low. As a result, it is difficult to obtain a desired refractive index.
The PbO component is one for stably forming a glass having a refractive index in the range from a comparatively low refractive index to a very high refractive index, by combining it with the SiO2. When the PbO component is present in an amount of less than 68%, the refractive index is low. As a result, it is difficult to obtain a desired refractive index. When it is present in an amount of over 78%, the refractive index is too high and the light transmittance deteriorates. Because the glass having an especially excellent light transmittance is easily obtained, the PbO component is preferably present in an amount of less than 73%.
The TeO2 component is an extremely important component in the present invention. The TeO2 component increases the refractive index of the glass, and decreases the melting temperature of the glass to improve the melting property. Further, the TeO2 component has an effect of improving the light transmittance of the glass, in particular, improving the light transmittance in a short wavelength band, dramatically. However, when the TeO2 component is present in an amount of less than 0.1%, the above effects are extremely low. Therefore, it is difficult to confirm the expression of the effects. When the TeO2 component is present in an amount of over 3.5%, on the contrary, the light transmittance of the glass deteriorates and a degree of coloring of the glass increases. It is difficult to obtain a desired transmittance. The glass is easily devitrified. It is difficult to obtain a homogeneous glass. Therefore, it is not preferable that the TeO2 component is present in an amount of over 3.5%. Because a raw material of the TeO2 component is expensive, the Teo2component is preferably present in an amount of not more than 2.5% in order to keep the excellent light transmittance and to save the manufacturing cost of the glass.
In the glass including the TeO2 component, the B2O3 component has an effect of increasing an amount of the TeO2 component to be compounded in the glass, and an effect of decreasing the melting temperature of the glass. Further, because the B2O3 component has an effect of improving the light transmittance of the glass, it can be optionally added. In order to obtain these effects, it is sufficient that the B2O3 component is present in an amount of not more than 6%. When it is present in an amount of over 6%, chemical durability (water resistance, acid resistance, weathering resistance and detergent resistance) of the glass deteriorates. Therefore, it is not preferable that the B2O3 component is present in an amount of over 6%.
The R2O component, that is, each component of Li2O, Na2O and K2O has an effect of promoting a melt of glass raw material and decreasing a melting temperature when compound glass raw material is melt. Therefore, each of these components can be optionally added. However, when one of these components is present in an amount of over 5%, or when two or more components are present in a total amount of over 5%, the chemical durability (water resistance, acid resistance, weathering resistance and detergent resistance) of the glass deteriorates. As a result, environment resistance which is required for a product, cannot be sufficiently kept. It is preferable that one of these components is present in an amount of less than 0.3%, or that two or more components are present in a total amount of less than 0.3%, because the glass having excellent chemical durability can be easily obtained. It is more preferable that that these components are present in a total amount of less than 0.1%, because the glass having more excellent chemical durability can be easily obtained.
By substituting the Rxe2x80x2O component, that is, one or more selected from the group consisting of SrO, BaO and ZnO, for a part of the PbO component, the refractive index of the glass can decrease and the dispersion can be suppressed lower. However, when one or more of these components are present in a total amount of over 5%, it is difficult to obtain a desired optical constant.
The GeO2 component can be optionally added in order to adjust an optical constant of the glass, and to improve chemical durability (water resistance, acid resistance, weathering resistance and detergent resistance) of the glass and resistance to devitrification property. However, when it is present in an amount of over 5%, the melting temperature of the glass increases and the melting property thereof deteriorates.
The Al2O3 component is effective to adjust the viscosity of the glass and to keep the chemical durability (water resistance, acid resistance, weathering resistance and detergent resistance) of the glass excellently. However, it is not preferable that the Al2O3 component is present in an amount of over 3%, because the glass is easily devitrified.
The Nb2O5 component is effective to enhance the chemical durability (water resistance, acid resistance, weathering resistance and detergent resistance) of the glass and to adjust the refractive index of the glass. When the Nb2O5 component is present in an amount of over 3%, the light transmittance of the glass deteriorates and a degree of coloring of the glass increases. Further, the glass is easily devitrified. Therefore, it is not preferable that the Nb2O5 component is present in an amount of over 3%.
Both In2O3 component and Ga2O3 component have an effect of increasing the refractive index. These components can be optionally added in case of necessity. When these components are present in a total amount of over 3.5%, the light transmittance of the glass deteriorates and a degree of coloring of the glass increases. A desired transmittance cannot be obtained. The glass is easily devitrified. Further, it is difficult to obtain a homogeneous glass. Therefore, it is not preferable that these components are present in a total amount of over 3.5%.
Both As2O3 component and Sb2O3 component are generally used as a refining agent. In order to obtain the refining effect, it is sufficient that each component is present in an amount of not more than 1%. In the glass according to the invention, because the As2O3 component has not only the refining effect but also an effect of improving the light transmittance of the glass, it is preferable to use the As2O3 component as a refining agent.
One or more fluorides which are partially or wholly substituted for one or more oxides of each metal element explained above, has an effect of improving the light transmittance of the glass and are effective to adjust the refractive index of the glass lower. However, in order to keep the resistance to devitrification property of the glass, the F elements of the fluorides should be present in a total amount of not more than 2%. It is not possible that the GeO2 component, the Al2O3 component, the Nb2O5 component and the F elements are present in a total amount of over 5%, because the resistance to devitrification property of the glass deteriorates and the glass is easily colored.
Next, in an optical glass according to the second aspect of the present invention, the reason why a composition range of each component is limited as described above, will be explained.
The SiO2 component is an essential component for forming a glass. However, when the SiO2 component is present in an amount of less than 18%, the refractive index of the optical glass is too high and the light transmittance is not sufficient. When the SiO2 component is present in an amount of over 27%, the refractive index becomes low. As a result, it is difficult to obtain a desired refractive index.
The PbO component is one for stably forming a glass having a refractive index in the range from a comparatively low refractive index to a very high refractive index, by combining it with the SiO2. Further, the PbO component is an essential component for minimizing the absolute value of the photoelastic constant (xcex2). However, when the PbO component is present in an amount of less than 71%, the photoelastic constant (xcex2) becomes large. As a result, a desired photoelastic constant cannot be obtained. Further, the refractive index is low. As a result, it is difficult to obtain a desired refractive index. When it is present in an amount of over 78%, the refractive index is too high and the light transmittance deteriorates. In order to easily obtain the glass having an especially excellent light transmittance, the PbO component is preferably present in an amount of not more than 73%.
The TeO2 component is an extremely important component in the present invention. The TeO2 component increases the refractive index of the glass, and decreases the melting temperature of the glass to improve the melting property. Further, the TeO2 component has an effect of improving the light transmittance of the glass, in particular, improving the light transmittance in a short wavelength band, dramatically. Further, the TeO2 component has an effect of minimizing the absolute value of the photoelastic constant (xcex2) in the SiO2xe2x80x94PbO system glass. However, when the TeO2 component is present in an amount of less than 0.2%, these effects are extremely low. Therefore, it is difficult to confirm the expression of the effects. When the TeO2 component is present in an amount of over 3.5%, on the contrary, the light transmittance of the glass deteriorates and a degree of coloring of the glass increases. It is difficult to obtain a desired transmittance. The glass is easily devitrified. It is difficult to obtain a homogeneous glass. Therefore, it is not preferable that the TeO2 component is present in an amount of over 3.5%. Because a raw material of the TeO2 component is expensive, the TeO2 component is preferably present in an amount of not more than 2.5% in order to keep the excellent light transmittance and to save the manufacturing cost of the glass.
In the glass including the TeO2 component, the B2O3 component has an effect of increasing an amount of the TeO2 component to be compounded in the glass, and an effect of decreasing the melting temperature of the glass. Further, because the B2O3 component has an effect of improving the light transmittance of the glass, it can be optionally added. In order to obtain these effects, it is sufficient that the B2O3 component is present in an amount of not more than 6%. It is not preferable that the B2O3 component is present in an amount of over 6%, because the chemical durability (water resistance, acid resistance, weathering resistance and detergent resistance) of the glass deteriorates. Further, because the B2O3 component has a tendency to increase the value of the photoelastic constant (xcex2), it is more preferable that it is present in an amount of not more than 3%.
The reason for limiting the composition range of each component other than the above components of the optical glass according to the second aspect of the present invention, is the same as that of each component of the optical glass according to the first aspect of the present invention. The explanation thereof is omitted.